Fragmented Paleogene foreland basin in the Valles Calchaquíes, NW of Argentina

HONGN, F.; DEL PAPA, C.E; PAYROLA, P.; PETRINOVIC, I. A.; MON, R.

Cenozoic Geology of the Central Andes of Argentina

SCS Publisher

Año: 2011; p. 189 - 209

Sedimentologic-stratigraphic, paleontologic and structural detailed surveys on the well-exposed Cenozoicsuccessions of theValles Calchaquíes have revealed key points for understanding the Paleogene CentralAndes evolution in NW-Argentina. Ourwork has documented several Eocene fossiliferous sections showingunconformable relationships with the underlying SaltaGroup and evidence of syndeformational depositionsuch as progressive unconformities (e.g. La Poma-Saladillo, Cerro Tin Tin,Luracatao). Our data along withthose offered by other research-groups have increased the Eocene foreland record, which displayssomeparticular characteristics: among them, most of the Paleogene faults were west- or bivergent, affected theNeoproterozoic-Lower Paleozoic basement defining a thick-skin deformation and followed a rather unsteadypropagation path. Evidence ofEocene deformation and sedimentation indicate that the Cenozoic shorteningin NW-Argentina is older than assumed in classicalmodels on Andean evolution proposing that crustalshortening was delayed until the late Oligocene-Miocene due to southwarddeformation migration from thePeruvian and Bolivian Andes. Our new data allow discussion of the Paleogene (Eocene) basinevolution,which is generally analyzed in the context of the classic foreland basin system. Basement blocks withdifferent gradesof exhumation compartmentalized the Eocene foreland. Thus it evolved as a broken forelandfrom its initial stages. Tectonicinversion and reactivation of both Lower Paleozoic basement heterogeneitiesand normal faults related to Cretaceous rift enhancedthe definition of the broken foreland and retrovergentstructures. The easternmost recognized Eocene structures (La Poma-Saladillo and Tin Tin) are westvergent.This back-vergence advances the hypothesis of an Eocene deformation front located evenfurthereastward. Precise definition of this deformation front is necessary to determine the eastern limit of theEocene deformationand whether the syntectonic deposits along the Puna-Eastern Cordillera transition fit inthe classical model of flexural forelandbasin related to tectonic load or whether alternative models have to beconsidered. Basement compartmentalization and lack ofwell-developed Eocene thrust belts westward (Punaand north Chilean Andes) suggest that alternative models should beexplored.